When is a Butler not a Butler ?

As far as I know the Butler is only used with crystals and particularly overtone crystals. It is essentially a Colpitts with a crystal added in the emitter feedback. The LC of the Colpitts defines the overtone so that the crystal operates at the correct overtone, but the high Q of the crystal (operated in non-reactive series mode) determines the actual operating frequency.

I think the defining characteristic of the Butler circuit is that the (series) tuned circuit is connected to the Cathode/Source/Emitter such that it forces the amplifying device's current to be a sinewave.

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Yes thats been my experience too, but I just heard it applied here to non series resonant types too.

Although Ive found a 2 transistor butler circuit is prefereable to a coplits at high overtones as you can use a much lower Q for the tank circuit so that the tank does not affect the crystal frequency so much.

Colin =^.^=

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Yes that would make sense, as you need a low impedance input node with a series tuned circuit. Its sometimes between the emitters of a differential pair.

Colin =^.^=

That is correct. That's what I remembered from the last one I built, back in the late '60's ;-) I Googled "Butler Oscillator" and found it right away.

Yep. Quite nice and easy to connect to other circuits without pulling.

...Jim Thompson

That's what comes to mind when I think of the Butler, but it seems like every time someone came up with a new oscillator scheme (ie a major change), someone else came along with a minor variant. Sometimes it's almost like those old "spot the difference" challenges in children's magazines, where the difference is pretty minor.

I don't know whether this is the case with the Butler, but with some oscillators, two defining points are needed to differentiate it from others.

Of course, older books tended to have a page or two of schematics for the major types of oscillators, making it easier to discern the differences, and people forget that people often put more effort into books than people put into webpages.

Michael

"Butler" was a Frank Butler of the RAF/GCHQ who published his new oscillator cct in "Wireless Engineer" ca 1944.

It was originally a cathode-coupled valve (tube) L-C oscillator and his published circuit used a pair of 6V6's. The big attraction of it was the simple parallel L-C circuit, so that freq band changes could be made with a inductance change, no reaction windings or taps being required.

However it's changed somewhat over the years, I first came across it in the Collins 75 series receivers, it used an xtal in series feedback between 2 halves of a 12AT7.

I think these days it applies to any oscillator that has a freq sensitive element between an emitter follower and a grounded-base amp.

Barry Lennox

Yes. The RSGB Radio Communication Handbook puts it like this: "the function of amplitude limiting is performed by a second stage, leaving the function of the first stage as an amplifier."

Change of subject. I'd like to know why Analog Devices dropped the elegant Butler JFET opamps from their lineup?

That would only apply if you aren't using AGC. They must be describing a specific implementation of the Butler.

I think it's been that definition since before the '60's when I ran across it and used it extensively.

It's fits nicely into IC's... easy to control amplitude so you don't saturate anything.

...Jim Thompson

You don't need AGC in a Butler, you can control amplitude with proper selection of the values in the "harmonic" tank.

...Jim Thompson

The OP275 ? doesnt mention it on the info page, I wonder what the butler part consists of as it doesnt show an internal schematic on the datasheet.

Colin =^.^=

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I don't know...Could it be the combination of the Collpitts and Armstrong oscillators, otherwise known as the ARM-PITS Circuit?

That's Butler, named after the inventor, James R Butler, see his 1992 patent # 5101126 conveniently downloaded at

The OP176 and SSM2275 opamp datasheets have schematics. He uses a bipolar input for low noise, plus a JFET input for high slew rates for input error voltages over 120mV. Butler was part of the Precision Monolithics team, SFAIK.

Huh???? You either need a nonlinear effect, like bottoming or the gm reduction with amplitude, or an AGC. If everything is linear and there is enough gain, the oscillation will grow until something starts to limit it.

AGC has the advantage over limiting that it doesn't cause the noise near harmonics to be aliased down to near the carrier.

The "Butlers" I've built have current sources in the bottom, not resistors ;-)

...Jim Thompson

In article , Jim Thompson wrote: [....]

Yes, and the transistors run from cut-off to conducting that much current. The nonlinearity in this case is the "gm reduction" type. The further into class C the stage goes, the lower its gm at the fundamental. It is still a nonlinearity that is limiting the amplitude.

"gm reduction" is in some ways a very good way to regulate the amplitude in an oscillator. Bottoming lowers the Q and because of saturation times pulls the frequency. The problem with "gm reduction" is that the amplitude varies from unit to unit more than an oscillator that uses bottoming.

It doesn't work that way. The right-hand current source is smaller than the left, so what is limited is drive current into a tank with limited (and known) Q.

...Jim Thompson